Force determining device



CURRENT (GURVE B) H. D. :sE N ERG FORCE DETERMINING DEVICE Filed Feb. 21, 1945 PRESSURE Y SOURCE w my 2 FE 7.0 Q u 8 a 2 E ct o:- 3 8 *7: 1 4 z E g a 0) LL 5 g 8 E g 2 G D g I I CONTACT PRESSURE= MILLIGRAMS (FORCE) 2mm M VIBRATOR CURRENT DEFLECTION FORGE DEFLEGTION DEFLEIGTIION I 0 31 CONTROLLED M DEVICE INVENTOR. HANS D. ISENEERG A TTORNEYS.

Patented Aug. 26, 1947 UNITED STATES PATENT OFFICE FORCE DETERMINING DEVICE Hans D. Isenberg, Wilmette, Ill.

Application February 21, 1945, Serial No. 579,139

The present invention relates to an improved device for producing an effect representative of a force and more particularly to improved apparatus for producing a variable effect which is accurately representative of a variable force.

There are many applications requiring accurate and rapid determination of a fixed or variable force. Thus in the measurement of torque and weight, determination of the force applied to a structural member is usually involved in obtaining an indication of the unknown factor. Commercially available measuring devices for determining such factors are predominantly of the allmechanical type, and conventionally comprise mechanical force translating systems for converting the factor under observation into an-effect, i. e., an indication or response, quantitativelyrepresentative of the factcn Such devices, if accurate, are expensive to manufacture and calibrate,

and frequently require tedious adjustments in the handling thereof in order to obtain accurate results, particularly in the measurement of small quantities. Moreover, the accuracy of such devices is impaired to a limited extent at least by device for producing an effect representative of a force. Q 7

It is another object of the'invention to provide a device of the character described which is very rapid in its response to changes in the force under observation, and is extremely accurate whenused to measure small forces.

It is still another object of the invention to provide improved apparatus of the character indicated which produces a response linearly related to the force under observation.

It is a still further object of the invention to provide an improved device of the character described, the accuracy of which is substantially unaffected by extraneous vibration of the device or by variations in the temperature of its compo 4 Claims. (01. 177-351) 2 istics of one of the elements embodied in the device;

Fig. 3 is a graph illustrating certain characteristics of another element of the device;

Fig. 4 schematically illustrates another embodiment of the invention in which the effect produced by the device is utilized for control purposes; and

Fig. 5 is a view schematically illustrating a further modification of the device in which facilities are provided for preventing extraneous vibration from afiecting the accuracy of the device.

Referring now to the drawings and more particularly to Fig. 1 thereof, the present invention is there illustrated in its embodiment in a device for measuring and indicating the pressure of a fluid pressure source [1, such, for example, as 'a steam generator or the like, having a pressure chamber which is fluid connected with a Sylphon bellows assembly l5 through a conduit [8. This assembly comprises the usual corrugated Sylphon element I50, the ends of which are closed by closure members I511 and 15b. The first of these members is fixedly mounted in any suitable manner to prevent movement thereof, and the other member I5!) is free to move back and forth along the longitudinal axis of the bellows element I50 in response to changes in the pressure within thi element.

In brief, the present improved device for measuring and indicating the pressure developed within the bellows elements I comprises a pair of electrically conductive contacts l0 and H, which are in the form of crossed rods of round cross section, having a contact resistance when engaged which is a predetermined function of the magnitude of the contact pressure therebetween. The contact H is rigidly mounted upon a conductive base I 4 which in turn is fixedly anchored against movement. Means comprising a rigid force transmitting member I6, upon which the contact I0 is fixedly mounted, are provided for producing a contact pressure between the .two identified contacts which is determined by the force exerted longitudinally of the member I6 to thrust the contact Ill against the contact ll.

In the illustrated arrangement, this force is obviously equal to the pressure per unit area within the fluid source 17 times the area of the bellows closure member I5b. If required, however, any

desired form of force dividing mechanism having a linear input force-output force characteristic may be interposed between the bellows member I51) and the contact Ill to produce a contact.

pressure betweenthe contacts 10 and I l which ;is 'fractionally and yet linearly related to the 3 pressure of the source ll. Such a mechanism may be necessary in certain applications in order to limit the pressure between the contacts l and l l to values within the usable portion of the contact pressure-contact resistance characteristic of the contacts in the manner explained below. The device further comprises electrical means controlled in accordance with the. contact resistance of the contacts Ill and H for-T producing an efiect, i, e., a visual indication, representative of the force applied longitudinally of the mem-" bar It by the bellows element 150. These electrical facilities comprise a voltage source I3; il-

lustrated as a battery of low voltage, and a galvanorneter 12 of the well known dynamometer type. Specifically, this galvanometer comprises a fixed coil 121), a movable coil HM, and an indicating element 520 adapted to coact with a suitable calibrated scale lZd to provide an indication of the magnitude of current flow through the series connected coils !2a and I2b. These coils are connected. in series .with thecontacts l0 and H across .the terminals" of the'voltage source l3, so that .the current flow therethrough is a function'of the contact resistance between the two identified contacts.

As. will be evident from the above description of the device, the fluid pressure within the source l! is translatedinto a force thrusting the con-,

tact it against the contact ll through the action of Sylphon bellows l5 and the force transmitting member to. Thus the contact pressure between In determining the curve A, a direct current voltage source of 1.2 volts was employed. Neglecting the external resistance of the circuit in which the contacts In and. I l are serially included, which resistance is very small as compared with the contact resistance of the contacts over the portion al--a2 of the curve A, the contact current-contact pressure characteristic B of the contacts, and hence of the circuit, is essentially the inverse of the described contact pressure-contact resistance characteristic. In other words, this characteristic curve also follows a square root law in that it satisfies the expression:

i i c where: V E=The applied voltage, and

I=Current through the contacts.

It is apparent, therefore, that if an instrument l2 having a linear current-indication response characteristic is usedto translate the current in "the indicating circuit into an indication of the force-thrusting the contact it! against the contact I l, non-linear calibration of the instrument scale 1201 must be relied upon to compensate for the v non-linearity of the curve B.

In accordance with a particular feature of the present invention, a linear relationship between the magnitude of the force thrusting the contact the contacts it and H is, determined by, and

varies in accordance with the force acting along the member I6, and hence the pressure of the source ll. At eachpredetermined value ofcontact pressure, .the conta cts lil and II have a corresponding contact resistance. J lccordingly, the current flow through the windings Ila and IZbof the galvanometer l2, and hence theidefiectionoi. the indicatingelement lZc along the scale l 2d,.is,determined by the pressure within the source I! and is variedin accordance with changes inthis pressure. By suitable calibration of the scale IZdf therefQre, the elements I20" and [2d are caused to coact in providing adirectly l0 against the contact H and the deflection of the galvanometer I2 is obtained by employing a galvanometer of the dynamometer type described above. 'This' instrument is a square law instrument in that the deflection of the indicating element is a function of the square of the current through the coils thereof. Ingeneral, the rela- J tionship between these iactors is defined by the readable indication of the pressure within the source H, or the force applied to the member-Iii.-

More specifically considered, the-configuration oi the contact resistance-contact pressure char acteristic of the crossed contact rods l0: and: I! a is typified by the curve A shown in Fig.2 "ofthe drawings. This curve is a quantitatively accurate representation of the contact resistance contact pressure characteristic for a pair oi crossed cor'i expression:

D=,KI

l 24 2 K K where; I v

D=Defiection of the instrument indicating element.

KriConstant of the instrument;

K=A new constant obtained by removing K from beneath the square root sign.

Relating the contact pressure between the contacts ll) and H to deflection of the instrument tact rods formed of brass andhaving "outside 1 diameters of .15625 inch and 322% inch, respectively. From an inspection of this'curve, it will be observed that within the portion ale-a2 of this curve which may be used in translating force into an effect or indication, the curve is essen tially non-linear. Specifically, this portion of the curve follows an inversesquare root law and accurately conforms to the expression;

. n/r I where: Y

R=Contact resistance in'ohmslf v P=Contact pressure in any unit weight; C=A constant which is dependent shape of the contacts, the material from which thecontacts are made,' the voltage applied across the contacts, and the system ofweights used in determining P.

" .76 the square root signs. upon the Since P is 'equal to the .force acting along the It is apparent, therefore, that when an instrument I2 having a square law current-deflection characteristic is employed, the relationship between the force thrusting the contact III against the contact I I and the resulting deflection of the instrument indicating element I20 becomes substantially linear. Graphically, the current-deflection characteristic of the instrument I2 as shown at C in Fig. 3, should correspond in pattern to the current-pressure characteristic B of the contacts I and II and hence is the inverse of the contact pressure-contact resistance characteristic A. Under such circumstances, the force-deflection characteristic of the device as indicated at D in Fig. 3 becomes absolutely linear. Hence a linearly calibrated scale I2d may be provided in the instrument I2 to produce an effect, 1. e., an indication, representative of the force acting along .the member I6.

In the arrangement illustrated in Fig. 4 of the drawings, the present improved device is utilized to effect response of a controlled device 21 when the force acting along the force transmitting member I6 reaches a predetermined value. In this embodiment of the device, the contacts I0 and II are connected in series with a biasing resistor 28 across the voltage source I3 to vary the bias potential between the input electrodes of a control tube 26. The space current path through this tube is arranged in series circuit relationship with the winding 3Ia of a control relay 3| and a source of space current 30. The relay 3| also includes an armature 3 lb for closing the normally open contacts 3Ic associated therewith when current floW through the Winding 3Ia reaches a predetermined value. A C battery 29 serially associated with the biasing resistor 28' between the input electrodes of the tube 26 is also provided normally to bias the tube to a threshold point at which current flow through the winding 3 Ia. is slightly less than that required to effect operation of the armature 3Ib to close the contacts 3Ic. In. the operation of the Fig. 4 arrangement, an increase in the force applied to the force transmitting member I6 to a predetermined value has the effect of increasing the contact pressure and hence lowering the contact resistance of the contacts I 0 and II to increase the current flow through the biasing resistor 28. The voltage drop through this resistor opposes the fixed voltage of the bias voltage source 29, so that when current flow through the resistor reaches a predetermined value representative of a predetermined force applied to the member I6, the space current flow between the output electrodes of the tube 26 is sufiicient to effect operation of the relay 3|. In operating, the armature 3Ib of this relay is actuated to close the contacts 3 I0 and thus complete an obvious circuit for energizing of the actuating winding of the controlled device 21 from the current source 32. When its winding is thus energized, the device 21 operates to perform its assigned control function. Illustrative of the use of the Fig. 4 embodiment of the invention, is the provision of an electromagnetic valve 21 to control the withdrawal of fluid from the source I! when the pressure of this source reaches a predetermined value.

One of the problems involvedv in maintaining the accuracy of the device when it is used to measure exceedingly small forces is that of preventing extraneous vibration of the device from changing the contact resistance of the contacts I0 and II within such wide limits as to preclude an accurate determination of the force under observation. Such extraneous vibration forces may be transmitted to the contacts III and II through the base I4 upon which the stationary contact I I is fixedly mounted or through the parts of the force transmitting mechanism comprising themember I6. In order'to obviate this difficulty, and to obtain an indication on the measuring instrument which is substantially independent of extraneous vibratory forces acting upon the contact assembly, the arrangement illustrated in Fig. 5 of the drawings is provided. As there shown, the contact supporting base I4 is fixedly mounted upon a platform 2I which in turn is spring mounted by means of coil springs 23 upon a supporting structure 22 subject to extraneous vibration. The platform 2| also fixedly supports an electro-mechanical vibrator 24 which is adapted for energization fro-m a suitable current source indicated by the bracketed terminals 25, and is utilized to oscillate the contact II at a fixed predetermined rate in the direction along which the contacts I0 and II are engaged. For maximum effectiveness in rendering the response of the device independent of extraneous Vibratory forces, the springs 23 should each have a very low spring constant, and the vibratory force produced by the vibrator 24 should have a frequency and amplitude substantially greater than the highest frequency and largest amplitude of any extraneous vibratory force acting upon the supporting structure 22. Preferably the vibrator 24 has a vibrating frequency of the order of 200 cycles per minute. Further, the natural resonant frequency of the moving system comprising the platform 2| and the parts carried thereby should be substantially higher than the highest frequency of any extraneous vibratory force acting upon the supporting structure 22. For reasons pointed out more fully below, it is preferable to use, in the Fig. 5 embodiment of the invention, an indicating instrument, such, for example, as a DArsonval galvanometer, having a linear current-deflection characteristic. An instrument I9 of this character is shown as having its moving coil connected in series with the contacts I0 and II and an adjustable calibrating shunt 20 across the terminals of the source I3.

As will be apparent from the preceding explanation, with the vibrator 24 in operation, vibratory forces are transmitted through the platform 2| and the contact base It which serve periodically to'vary the contact pressure between the contacts II! and II at the predetermined rate of vibration of the vibrator. Such forced vibration of the platform 2| and the springs 23 minimize the effects of the spurious vibratory forces acting upon the supporting structure 22. Thus the contact pressure produced by the force acting along the member I6 is periodically varied about a mean value representative of this force, through the action Of the vibratory force transmitted to the contact II by the vibrator 24.. pressure between the contacts If! and I I and hence the contact resistance therebetween is, therefore, rendered substantially independent of the extraneous vibratory forces. Further, since this contact pressure is varied, through the'action of the vibrator 24, about a mean value which is accurately representative of the force under observation and applied to the member I6, the contact resistance between the contacts II] and II is correspondingly varied about a mean value which is determined by the forceunder observation and is substantially independent of the extraneous vibratory forces. As a result, a direct current having a pulsating component is caused to flow in the circuit including the moving coil The contact 7 of the galvanometer [9. As is characteristic of such devices, the galvanometer i9 is essentially an' integrating instrument, such that the indicating element assumes a position accurately representative of the mean value of the current traversing its moving coil. Thus, the indicating element of the instrument assumes a setting which is accurately indicative of the force acting upon the contact l through the structural member l6, Suitable calibration of the instrument scale may be employed to provide for direct reading of the force applied to the member H5. The reason for providing an instrument l9 having linear current-deflection characteristic now becomes apparent. Thus, if an instrument of the character of the galvanometer i2, having a square law current-deflection characteristic, is employed, the indicating element of the instrument tends to assume a setting representative of the root mean square value of current traversing the coil of the instrument, rather than a setting representative of the mean Value of current flow therethrough. Thus, an inaccurate indication would be provided by the indicating element of such an instrument, in the absence of peculiar calibration. By providing an instrument I 8 having the described linear current-deflection characteristic, however, this problem is obviated. Further, this type of instrument is well suited to the use of an adjustable shunt 20 serially included in the circuit of its moving coil for calibration purposes or to provide for scale multiplication which permits different force ranges to be measured with the same instrument.

While there have been described what are at present considered to be the preferred embodiments of the invention, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention as defined in the appended claims.

1, Apparatus for producing an effect representative of a force, comprising a pair of electrically conductive contacts having a predetermined contact pressure-contact resistance characteristic when engaged capable of being represented by a curve other than a straight line, means for producing a'contact pressure between said contacts which is determined by said force, a current responsive device for producing said effect having a current-effect characteristic which is capable of being represented by a curve other than a straight line and which characteristic is substantially the inverse of said contact pressure-contact resistance characteristic, and means for passing a current through said device which varies in accordance with variations in the contact resistance between said contacts, thereby to produce a response of said device which is representative of said force.

2. Apparatus for producing an effect representative of a variable force, comprising a pair of electrically conductive contacts having a predetermined contact pressure-contact resistance characteristic when engaged capable of being represented by a curve other than a straight line, means for producing a contact pressure between said contacts which varies in accordance with variations in said force, current responsive means for producing said effect, and means for passing a current through said current responsive means which Varies in accordance with variations in the contact resistance between said contacts, thereby to produce a response of said current responsive means which is representative of said force, said current responsive means having a current-effect characteristic which is substantially the inverse of said contact pressure-contact resistance characteristic, whereby the response of said current responsive means bears a substantially straight line relationship with respect to variations in said force.

3. Apparatus for producing an efiect representative of a variable force, comprising a pair of electrically conductive contacts having a square law contact pressure-contact resistance characteristic when engaged, means for producing a contact pressure between said contacts which varies in accordance With variations in said force, current responsive means for producing said effeet, and means for passing a current through said current responsive means which varies in accordance with variations in the contact resistance between said contacts, thereby to produce a response of said current responsive means which is representative of said force, said current responsive means having a square law currenteffect characteristic which is substantially the inverse of said contact pressure-contact resistance characteristic, whereby the response of said current responsive means bears a substantially straight line relationship with respect to variations in said force.

4. Apparatus for producing an effect representative of a variable force, comprising a pair of electrically conductive contacts having a square law contact pressure-contact resistance characteristic when engaged, means for producing a contact pressure between said contacts which varies in accordance with variations in said force, and a circuit including said contacts and means controlled by the current in said circuit for producing an effect representative of said force, said circuit having a square law current-effect characteristic which is substantially the inverse of said contact pressure-contact resistance characteristic, whereby the response of said last-named means bears a substantially straight line relationship with respect to variationsin said force.

HANS D. ISENBERG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 749,854 Fahrney Jan. 19, 1904 1,814,465 Becq July 14, 1931 1,695,295 Rollins Dec. 18,- 1928 2,210,685 Oman Aug. 6, 1940 1,131,202 Boyden Mar. 9, 1915 FOREIGN PATENTS Number 7 Country Date 555,686 Germany July 29, 1932 OTHER REFERENCES (British), July 1945, page 287 (201-481). (Copy available in Div. 60, Patent Ofiice.) 

